1. Field of the Invention
The present invention is generally in the field of semiconductor devices. More specifically, the present invention is in the field of semiconductor amplifiers.
2. Background Art
Power amplifiers, such as collector-voltage controlled power amplifiers, are used in a number of applications, including mobile telephones, for receiving and amplifying a radio frequency (RF) input signal and generating an RF output signal. The output power of a collector-voltage controlled power amplifier is basically set by fixing the voltage, such as VCC, on the collector of a power output transistor in one or more stages of the power amplifier. Since the output current drawn by the power amplifier is determined by VCC divided by the load impedance presented to the power amplifier by an output matching network, the output current is inversely proportional to the load impedance, which is assumed to remain constant under normal conditions. However, under adverse voltage standing wave ratio (VSWR) conditions, the load impedance presented to the power amplifier by the output matching network can significantly decrease, which can cause a corresponding undesirable increase in output current that can damage the power amplifier.
By way of background, VSWR can be defined as the ratio of the maximum radio-frequency (RF) voltage to the minimum RF voltage along a transmission line coupled to an antenna. In mobile telephones, the transmitter, including the power amplifier, matching network, and switcher/diplexor, may be specified to withstand a high VSWR at the antenna without damaging the circuitry. For example, a mobile telephone may be specified to withstand a VWSR of 20:1 at the antenna. Assuming an insertion loss of approximately 1.5 dB due to the matching network and switcher/diplexor, the VSWR at the output of the power amplifier may be reduced to approximately 4.5:1, which can cause the load impedance to be reduced by a factor of 4.5. As a result, the output current of the power amplifier can undesirably increase, which can drain the battery of the mobile telephone and damage the power amplification circuitry and the voltage regulatory circuit coupled to the power amplifier.
Thus, there is need in the art for a circuit that can prevent a decrease in load impedance at the output of a power amplifier, such as a collector-voltage controlled power amplifier, from causing an undesirable increase in current drawn by the power amplifier.